This invention relates to the construction of vacuum envelopes, and more particularly, to improved tubulation used in sealing a vacuum envelope.
In the production of vacuum envelopes such as those used in housing electronics components, it is common practice to braze a ductile tubulation to an exhaust passage in the wall of the envelope. This tubulation is normally a cylindrical copper or glass tube having a circular cross-section. After exhausting the envelope through the tubulation, the tubulation is pinched-off and a vacuum seal is created by the pinched walls of the tube. A similar process is used in manufacturing a gas-filled enclosure where the envelope is backfilled rather than evacuated.
In the manufacture of such vacuum envelopes, it is desirable to use as large a tubulation as possible in order to provide for greater throughput and faster vacuum pumpdown. It is also desirable to locate the point of pinch-off as close as possible to the envelope wall in order to reduce the likelihood of tubulation breakage in handling the finished envelope and to minimize the overall size of the completed package.
Problems can arise however, with the use of large bore cylindrical tubulation when the point of pinch-off is close to the point at which the tubulation is brazed to the envelope. That is, the closer the point of pinch-off in relation to the braze interface, the greater the stress on the braze during pinch-off. This stress can cause the walls of the tubulation to break away from the envelope resulting in a breakdown of the vacuum. While these stresses may be minimized through the use of small bore tubulation, this causes an increase in the time required for vacuum pumpdown. Stresses may also be minimized even for large bore tubulation by locating the point of pinch-off further away from the braze interface. However, in this case, the projecting portion of the tubulation remaining after pinch-off is more prone to breakage than one which is almost flush with the surface of the envelope. Further, this large projection is often unsuitable for use in miniature vacuum components where package size dictates the amount of acceptable projection.